1. Field of the Invention
This invention generally relates to polymeric materials and, more particularly, to a chemical process for producing powders of ketone-containing aromatic polymers (polyarylketone polymers) of micron and submicron particle size.
2. Description of the Prior Art
Polyarylketone polymers are tough, semi-crystalline, poorly soluble materials with high melting temperatures. The crystallinity contributes to the excellent chemical resistance, mechanical properties, and extended use temperature above the glass transition temperature (Tg). This combination of material properties is desirable for engineering and structural applications. Typical examples of these materials include polyarylene ether ketone (PEK), polyarylene ether ether ketone (PEEK), polyarylene ether ketone ketone (PEKK), etc.
Polyarylketone polymers are produced by a number of different processes. The two traditional routes for the polymerization of these polymers are aromatic electrophilic substitution and aromatic nucleophilic substitution. The electrophilic substitution method suffers from the fact that it produces ortho substitutions as well as the desired para linkages. The conventional nucleophilic displacement reaction requires the use of high boiling solvents such as diphenylsulfone to prevent premature crystallization, and the removal of solvent and salt byproducts from the polymer has proven extremely difficult. In addition, the high reaction temperatures required (280.degree.-320.degree. C.) may lead to unwanted side reactions such as branching.
The production of linear polymers of semicrystalline polyether ketones without side reactions has been investigated by a number of researchers. Mohanty et al., 31st Int. SAMPE Symp., 31:945 (1986), and Risse et al., Macromolecules, 23:4029 (1990), disclose a method to produce high molecular weight amorphous polymers by the incorporation of removable bulky alkyl susbstituents (e.g., t-butyl) along the polymer backbone. The alkyl groups are cleaved from the polymer backbone by strong acids to generate the semicrystalline polyether ketones. Cleavage of the alkyl groups is relatively slow (&gt;20 hours) and requires the use of trifluoromethane sulfonic acid as a solvent. Kelsey et al., Macromolecules, 20:1204 (1987), discloses reacting the ethylene glycol acetal of dihydroxybenzophenone with difluorobenzophenone to form poly(ketal ketones). These poly(ketal ketones) were subsequently hydrolyzed to the polyether ketones. The Kelsey et al. reaction process is limited in its applicability to bisphenol monomers which contain ketone moieties since the ketal is not an electron withdrawing group and cannot activate a dihalide monomer toward polymerization. Mohanty et al., 32nd Int. SAMPE Symp. 32:408 (1987) discloses reacting 4,4'-difluorobenzophenone in the presence of molecular sieves with aniline to produce a ketimine derivative, when polymerized with a bisphenolate produces an amorphous poly(arylene ether ketimine). The ketimine is subsequently cleaved with dilute acid to regenerate the ketone.
The chemical and solvent resistance of polyarylketone polymers along with their high melting temperature (Tm) make processing of this material quite difficult. For example, impregnating carbon fiber pre-pregs with polyarylketones to form composite materials useful in engineering structures is presently performed by a number of time consuming and expensive techniques. There are several solvent based processes in which the carbon fibers are drawn through a solution of resin. With this type of method, one must be able to dissolve the polymer. Polyarylketone polymers such as PEEK require the use of high temperatures and exotic solvent systems to dissolve the polymer. The harsh conditions employed often break the carbon fibers during impregnation. Furthermore, the solvent systems employed are often toxic and difficult to isolate, as well as pose an environmental hazard. Another approach is to produce a continuous fiber of the polymer and to weave or commingle these fibers with the carbon reinforcement. For this approach, one must be able to produce the polymeric fiber and have a means for mixing these fibers with the carbon fibers and these requirements both involve major technical challenges and capital investments.
Recently, much effort has been made in preparing powders from engineering polymers. The powders can be applied onto a fiber by electrostatically charging the polymer powder and depositing it onto the fiber via a fluidized bed, or by pulling the fiber through a suspension, preferably aqueous, of the powder. After the powder is applied, heat is employed to fuse the powder to the fiber and to consolidate the powder. Powder prepregging has been investigated for LARC-TPI (see, Baucom et al., 35th Int. SAMPE Symp., 35:175 (1990), Marchello, 36th Int. SAMPE Symp., 36:68 (1991), and Hou et al., 35th Int. SAMPE Symp., 35:1594 (1990)) and epoxy (see, Yang et al., 36th Int. SAMPE Symp., 36:1523 (1991) ). The particle sizes reported for powder prepregging range from 5 .mu.m (see, Thorne et al., 35th Int. SAMPE Symp., 35:2086 (1990)) to 80 .mu.m (see, Hedrick et al., 35th Int. SAMPE Symp., 35:82 (1990)).
Presently, powders of polymeric materials are made by grinding the polymer. Grinding is very expensive and also imposes mechanical limits on the size of particles which can be obtained.